1. Field of the Invention
The present invention relates to an oxidation catalyst and an exhaust-gas purification system using the same, and in particular, to an exhaust-gas purification system having excellent performance in purifying particularly nitrogen oxides from an internal combustion engine such as a diesel engine.
2. Description of Related Art
As a method for purifying nitrogen oxides discharged from an internal combustion engine such as a diesel engine, a method of installing a reduction catalyst along the flow of an exhaust gas and purifying NOx on a reduction catalyst by a reducing agent supplied from front of the reduction catalyst has drawn attention. Ammonia (NH3), urea ((NH2)2CO), a hydrocarbon such as light oil, and the like are used as the reducing agent. It is known that the reaction for purifying NOx proceeds via NO2 using any of the above reducing agents. It is known, for example, that the reaction for purifying nitrogen oxides using urea proceeds mostly according to following formulae (1) to (4) and that formula (2) known as standard-SCR (selective catalytic reduction) is a slower reaction than formula (3) known as fast-SCR(C. Scott Sluder, et al., Low Temperature Urea Decomposition and SCR Performance, SAE paper 2005-01-1858).
Most of the nitrogen oxides in the exhaust gas discharged from an engine are NO when the engine is running at a low output (low speed) (that is, when the exhaust gas is at low temperature), while the equimolar amounts of NO and NO2 react in formula (3). It is considered, therefore, that the reaction of formula (3) is promoted by making the molar ratio of NO2 to NO close to 1:1 by forming NO2 when the exhaust gas is at low temperature.(NH2)2CO+H2O→2NH3+CO2  (1)4NH3+4NO+O2→4N2+6H2O  (2)2NH3+NO+NO2→2N2+3H2O  (3)2NO2+4NH3+O2→3N2+6H2O  (4)2NO+O2→2NO2  (5)
A method of making use of plasma is known as a technology for forming NO2. A method for decreasing particulate matters in an exhaust gas by using nitrogen dioxide and ozone formed by means of generating plasma in the exhaust gas is disclosed (US-A-2004-168429 (JP-A-2004-169643)).
Another technology for forming NO2 is a technology of using an oxidation catalyst to promote the reaction of the above formula (5). For example, platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh) and the like are proposed as a active component of a catalyst for oxidizing NO in the exhaust gas to NO2 in pursuit of removing diesel particulates (particulate matters) in U.S. Pat. No. 4,902,487 (JP-A-1-318715). Specific data on the metals other than Pt, however, are not disclosed. In addition, in “Barry J. Cooper, et al., Role of NO in Diesel Particulate Emission Control, SAE paper, 890404”, it is shown that Pd, Rh, iridium (Ir) and Ru catalysts have poor performance of NO oxidation and in particular that Pd supported on Al2O3 has a NO oxidation ratio of 0%.
On the other hand, as purification method of nitrogen oxides, a purification method for an exhaust gas in which a part of NO is oxidized to NO2 with a Pt catalyst and then the exhaust gas is introduced to on a reduction catalyst together with ammonia, noticing that most reduction catalyst has higher activity for nitrogen dioxide than for nitrogen monoxide, is proposed in US-A-2002-039550 (JP-A-2002-1067).